Hi Marie, sometimes you can say "pazzesco" (crazy, absurd) questo film è pazzesco - this movie is crazy if you want to use a very slang word you can also say "fico" (or figo, "c" or "g" depends on local pronunciation), usually in english it means cool. il mio viaggio è stato davvero fico - my journey has been really awesome But avoid to use (better never use) the feminine version of this word, it's a really vulgar and rude word. Anyway, it's better to use the words suggested by Andre in his comment.

Stupendo or meraviglioso but they are formal :)

Fantastico! Mitico! Grandioso! Meraviglioso! Incredibile! I hope it helps!

Su duolinguo usano " figo" termine usato ultimamente dai giovani ma non propriamente elegante anche se fa italian slang. Io userei imponente, maestoso oppure pauroso ( in senso positivo )

We need a context to give you a useful answer, otherwise it would be copying a dictionary entry.